Electrical activity in circadian clock neurons plays an important role in cellular oscillation. However, the particular voltage-gated ionic conductances necessaryfor circadian function and the mechanisms by which electrochemical signals interact with transcriptional events in the nucleus of clock neurons remain unknown. To identify the particular ionic conductances necessary for cellular oscillation, ion channel subtype-specific membrane-tethered toxins are transgenically expressed in clock neurons to interfere with voltage-gated conductances. Preliminary studies reveal substantial effects of in vivo expression of Ca2+ and K+ channel tethered toxin blockers on free-running behavioral rhythms. The proposed aims are (1) to test dose- dependence of these behavioral effects, (2) to examine effects of tethered toxins on cellular oscillation, and (3) to determine the molecular identities of the blocked ion channels. The long-term goal of this proposal is to improve understanding of circadian clock function, an issue of substantial health relatedness, given the great costs to society of genetic and environmental disruption of human sleep/wake cycles.